Recently, there are more and more opportunities to store audiovisual data including audio data and video data, which have been broadcast either as airwaves or over a telecommunications network, on optical discs. As the size of the data to be stored has been increasing, there is an increasing demand for optical discs with high storage density and huge storage capacity.
To store data at a higher density on an optical disc, the beam spot of a light beam for use to perform a read/write operation needs to have a decreased size. For that purpose, an objective lens with a large numerical aperture (NA) is used more and more often.
However, the greater the numerical aperture, the closer to the information storage layer of an optical disc the objective lens should be located. In that case, if the optical disc had a significant tilt due to warp, for example, then the influence of coma aberration to be produced in such a situation would be no longer negligible. This is because the coma aberration would interfere with the drive's proper read and write operations.
Some conventional optical heads for optical discs have the ability to detect the relative angle defined by the given optical disc with respect to the optical axis of the objective lens (such an angle will be referred to herein as a “tilt”). If the tilt detected is corrected such that the optical axis crosses the information storage layer of the optical disc at right angles, then the coma aberration will not be produced.
FIG. 15 schematically shows a configuration for a conventional optical head and conventional tilt detecting mechanism that are disclosed in Patent Document No. 1. The conventional optical head shown in FIG. 15 includes a light source 101, a beam splitter 103, an objective lens 104, a detection optical system 106, a photodetector section 107, and a tilt detecting section 108. The tilt detecting section 108 includes a signal computing section 109, an amplifier 110 and a differential amplifier 111.
The light emitted from the light source 101 and having an optical axis 102 is transmitted through the beam splitter 103 and then converged by the objective lens 104 onto the information storage layer of a given optical disc. Next, the light is reflected from the optical disc 105, transmitted through the objective lens 104 and then reflected by the beam splitter 103 toward the detection optical system 106. Thereafter, the light is transmitted through the detection optical system 106 and then incident onto the photodetector section 107.
FIG. 16 schematically illustrates the photodetection areas of the photodetector section 107 and the cross section (i.e., beam spot 112) of the incident light beam. The photodetector section 107 includes photodetection areas 107a through 107f, which are irradiated with the light beam that has been reflected from the optical disc 105 and then has formed the beam spot 112 there.
As shown in FIG. 16, the light beam with the beam spot 112 is detected in the six divided photodetection areas 107a through 107f. Specifically, the two arc portions, which are located on the right- and left-hand sides of the beam spot 112, represent areas in which the zero-order and ±first-order components of the light that has been diffracted by the information track groove of the optical disc 105 superpose one upon the other. Also, the arrow shown in FIG. 16 indicates the directions in which the information track runs on the optical disc 105.
The signals generated in the photodetection areas 107a through 107f are supplied to the signal computing section 109, which generates differential signals P01 and P02 by performing the following calculations on the supplied signals:P01=107a−107b P02=(107c+107e)−(107d+107f)where the reference numerals 107a through 107f represent the signals that have been generated in the respective photodetection areas 107a through 107f. 
In the tilt detecting section 108, the amplifier 110 multiplies the differential signal P02 by k0. Thereafter, the differential amplifier 111 subtracts the product from the differential signal P01 and outputs the difference as a tilt detection signal TL0. That is to say, the tilt detection signal TL0 is given by:TL0=P01−k0×P02The constant k0 is determined so as to correct the offset of the differential signal P01, which is caused by the shift of the optical axis of the objective lens 104 from the optical axis 102 of the optical head, by that of the differential signal P02. Thus, the tilt detection signal TL0 would be a signal that is unlikely to be affected by the shift of the optical axis of the objective lens 104.
If the optical disc 105 has tilted in its radial direction with respect to the optical axis 102 of the optical head (i.e., if a so-called “radial tilt” has been produced), then its influence will come up as a difference in phase between the differential signals P0 and P02. This is because a coma aberration is produced while the laser beam is being transmitted through the transparent substrate of the optical disc 105. The coma aberration mainly deforms the wavefronts of portions of the light beam in which the zero-order and ±first-order components of the light diffracted by the information track superpose one upon the other. As a result of the generation of the coma aberration, the differential signals P01 and P02 are subject to different types of modulation by the information track. Consequently, a phase shift is produced between the differential signals P01 and P02.
The tilt detection signal TL0, which is calculated by subtracting k0×P02 from the differential signal P01, will also cause a phase shift due to the radial tilt. That is why by detecting the level of the tilt detection signal TL0 while the beam spot is tracing the center of the information track, the radial tilt can be detected.
In an optical disc of a so-called “amplitude modulation type” (such as a phase change type) on which an information track where information is stored and another information track where no information is stored have mutually different reflectances, the symmetry of the light intensity distribution at the center of a light beam spot might change significantly due to a variation in the converging state toward the optical disc (which will be referred to herein as “defocusing”) on the boundary between the recorded track and the unrecorded track (which will be referred to herein as a “recording boundary”). As a result, an offset is produced in the tilt detection signal, thereby causing a tilt detection error.
To overcome such a problem, an optical head for optical discs, having the ability to reduce the offset in the tilt detection signal at the recording boundary, has been developed. FIG. 17 shows the photodetection areas of the photodetector section 107′ of the conventional optical head disclosed in Patent Document No. 2 and a light beam spot of the light beam that has been incident on the photodetection areas. The elements of the optical head disclosed in Patent Document No. 2 are the same as the counterparts of the optical head shown in FIG. 15 except the photodetector section 107′. Thus, the following description will be focused on the photodetector section 107′.
FIG. 17 illustrates the beam spot 112′ of the light beam that has been incident on the photodetector section 107′ and photodetection areas 117a through 117f that are divided to detect the intensities of the light beam received at the respective areas. The light beam is received at the six divided photodetection areas 117a through 117f to leave the beam spot 112′ that covers those areas. Then, the signal computing section 109 (see FIG. 15) figures out two differential signals P01 and P02 by the following equations:P01=117a−117b P02=(117c+117e)−(117d+117f)And the signal TL0 is given by:TL0=P01−k0×P02where k0 is a constant.
In FIG. 17, an opaque portion 113 is arranged at the center of the beam spot 112 where the symmetry of the light intensity distribution changes significantly (i.e., an area where there are only zero-order components of diffracted light). As a result, the influence of the symmetry variation at the recording boundary on the differential signals P01′ and P02′ can be reduced.
It is known that by optimizing the dimensions and shapes of the respective photodetection areas, not just the offset to be caused by the displacement of the objective lens but also the offset produced at the recording boundary can be corrected with the correction coefficient k0 simultaneously and the detection error can also be reduced as well.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2003-45058    Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2005-141893